The present invention relates generally to the conditioning of carbonaceous electrodes for electrochemical systems, and particularly to a method of activating porous graphite as halogen electrodes in a metal-halogen battery.
Electrochemical devices or systems of the type referred to herein include one or more of the metal-halogen battery systems, such as molten salt metal-chlorine battery systems, and may be applicable to other non-aqueous batteries which use carbonaceous electrodes. The metal-halogen battery systems generally are comprised of electrode stacks and other auxiliary subsystems for suitable battery operation. The battery cell is comprised of positive and negative electrodes which are both in contact with a molten salt of a suitable metal halide composition. In the lithium-aluminum/chlorine battery system, chlorine gas is liberated from the positive electrodes of the cell and may be stored either on the electrode as adsorbed chlorine or separate from the electrode stack.
With reference to the general operation of a lithium-aluminum/chlorine battery system, during charge lithium chloride - potassium chloride molten salt electrolyte is electrolyzed to form chlorine at the positive "chlorine" electrode in the cell. This chlorine electrode is typically made of a porous carbonaceous material. At the opposing negative or lithium-aluminum or lithium-alloy electrode, lithium metal is deposited which, in turn, forms an alloy' Literature cites a composition range of 10 atom % to 48 atom % of lithium in aluminum as a suitable operating range. Other lithium alloys such as lithium-aluminum-silicon, lithium-magnesium-silicon, lithium-silicon have been cited.
During the discharging of the lithium-aluminum/chlorine battery, chlorine is reduced at the chlorine electrode to chloride ions and lithium metal is oxidizied to lithium ions forming the lithium chloride salt and power is available at the battery terminals.
Over the course of the lithium-aluminum/chlorine battery charge/discharge cycle, the concentration of the electrolyte molten salt varies as a result of the electrochemical reactions occurring at the electrodes in the cell. At the beginning of charge, the concentration of lithium chloride-potassium chloride molten salt may typically be 68 mole % lithium chloride (LiCl) and 32 mole % potassium chloride (KCl). As the charging portion of the cycle progresses, the molten salt concentration will gradually decrease with the depletion of lithium and chloride ions from the molten salt. When the battery system is fully charged, the lithium chloride concentration in the molten salt will typically be reduced to 42 mole % LiCl with 58% KCl. Then, as the battery system is discharged, the electrolyte molten salt concentration will gradually swing upwardly and return to the original concentration when the battery system is completely or fully discharged.
The present invention is directed to an improved method of activating carbonaceous electrodes for primary or secondary battery systems in which a gaseous or liquid reactant is reacted at a carbonaceous electrode. The term "activating" used in this specification in connection with electrodes refers to a process of increasing the rate of the electrochemical reaction at a given overvoltage of the electrode. Similarly, the term "overvoltage" as used herein means for excess voltage above the normal reversible electrode potential of a carbonaceous electrode required to produce a desired rate of an electrochemical reaction or electrical current. In other words, by "overvoltage" is meant the difference between the electrode potential necessary to sustain an electrochemical reaction and the thermodynamic reversible electrode potential. One process for improving electrode activity is described in the Hart U.S. Pat. No. 4,120,774 entitled "Reduction of Electrode Overvoltage." This patent employs a thermal treatment of electrodes with nitric acid for a long period of time to achieve satisfactory activation. Another process for activating electrodes is described in the Carr, U.S. Pat. No. 4,273,839, entitled "Activating Carbonaceous Electrodes." This patent describes an electrolytic activation process which employs an aqueous electrolyte containing a current carrying water soluble material, and numerous examples of suitable inorganic current carrying water soluble materials are disclosed therein. The specific teachings of these patents are incorporated herein by reference.
It is a principal object of the present invention to provide an improved method of activating a carbonaceous electrode by electrolysis.
It is a more specific object of the present invention to provide a method of activating porous graphite electrodes in a metal-halogen battery cell, including a porous graphite positive electrode and a suitable negative electrode spaced apart from each other.
In order to achieve the foregoing objects, the present invention provides a novel method of activating a carbonaceous electrode, which comprises the steps of providing a negative electrode and a carbonaceous positive electrode spaced apart from each other, heating the electrodes to a desired temperature, removing surface debris from said electrodes, providing an aqueous electrolyte having a predetermined concentration of sulfuric acid therein, passing an electrical current of a predetermined current density through the positive and negative electrodes for a predetermined time period sufficient to develop an electrode pore structure so as to decrease the overvoltage of the positive electrode, rinsing said electrodes, and heating said electrodes at a desired temperature for a desired period of time to cause removal of the so-called activation layer from the electrodes.
Additional advantages and features of the present invention will become apparent from a reading of the detailed description of the preferred embodiment which makes reference to the following set of drawings in which :